nixos/bcache: add services.udev.packages = [ bcache-tools ]

Adds /dev/disk/by-{id,label}/* symlinks for bcache device nodes, in the
final rootfs.

Symlinks will only be created for bcache devices that contain
filesystems. So if you have a blank bcache device or run LVM on top of
bcache you will not get this kind of symlink.

(cherry picked from commit 5b48368)

nixos/modules/tasks/bcache.nix

@@ -4,6 +4,8 @@
 
   environment.systemPackages = [ pkgs.bcache-tools ];
 
+  services.udev.packages = [ pkgs.bcache-tools ];
+
   boot.initrd.extraUdevRulesCommands = ''
     cp -v ${pkgs.bcache-tools}/lib/udev/rules.d/*.rules $out/
   '';